<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // 1. 用户输入
    let year = +prompt('请输入年份')
    // 2. 判断输出
    if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
      alert(`${year}年是闰年`)
    } else {
      alert(`${year}年是平年`)
    }
  </script>
</body>

</html>